In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possesing enhanced thermal stability. Such modification has generally been accomplished by heating a fibrous material in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.
U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion possessing fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of shaped articles of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) a chain scission and oxidative cross-linking reaction of adjoining molecules, as well as (2) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, in the past the stabilization reaction must by necessity at least initially be conducted at relatively low temperatures (i.e. below about 300.degree. C.), since the cyclization reaction is known to be exothermic in nature and must be controlled if the original configuration of the material undergoing stabilization is to be preserved. Accordingly, the stabilization reaction has tended to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes proposed to shorten the period required by the stabilization reaction include that disclosed in U.S. Pat. No. 3,416,874. See also the process of our commonly assigned U.S. Pat. No. 3,592,595, wherein the cyclization of pendant nitrile groups of the acrylic fibrous material is catalytically enhanced while the fibrous material is immersed in a solution of a Lewis acid at a temperature of about 160.degree. to 260.degree. C.
While stabilized acrylic fibrous materials may be used directly in applications where a non-burning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in an inert atmosphere, such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term "carbonized" as used in the art commonly designates a material consisting of at least about 90 percent carbon by weight, and generally at least about 95 percent carbon by weight. Depending upon the conditions under which a carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic x-ray diffraction pattern of graphite. See, for instance, commonly assigned U.S. Ser. No. 777,275, filed Nov. 20, 1968 (now abandoned) of Charles M. Clarke for a preferred procedure for forming continuous lengths of carbonized and graphitized fibrous materials from a stabilized acrylic fibrous materials.
It is an object of the invention to provide an improved process for enhancing the thermal stability of a shaped acrylic article.
It is an object of the invention to provide an improved process for the flame-proofing of a fibrous material or film formed from an acrylic polymer to produce a dimensionally stable flexible product.
It is an object of the invention to provide a process for the thermal stabilization of an acrylic fibrous material or film wherein degradation and chain scission within the acrylic precursor is substantially diminished.
It is an object of the invention to provide a process wherein the thermal stabilization of an acrylic fibrous material or film optionally may be conducted for a brief residence time at a more highly elevated temperature than heretofore commonly utilized.
It is another object of the invention to provide an improved process for the stabilization of fibrous materials or films formed from acrylic polymers which results in a product which is suitable for carbonization, or carbonization and graphitization.
It is further object of the invention to provide a process for converting an acrylic fibrous material or film to a stabilized form possessing substantially the identical configuration as the starting material.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.